This was an open fixme for some time. ubiformat does not care about used
ubi volumes or attached ubis.

This patch adds functionality that umounts all filesystems that are
mounted from this nand device. After that the ubi is detached. Then the
normal ubiformat code reformats the ubi. If a ubi was detached
previously, the code tries to reattach the ubi. Filesystems are not
remounted.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>